Fuel cells are devices that directly convert chemical energy of reactants, i.e., fuel and oxidant, into direct current (DC) electricity. For an increasing number of applications, fuel cells are more efficient than conventional power generation, such as combustion of fossil fuel, as well as portable power storage, such as lithium-ion batteries.
In general, fuel cell technology includes a variety of different fuel cells, such as alkali fuel cells, polymer electrolyte fuel cells, phosphoric acid fuel cells, molten carbonate fuel cells, solid oxide fuel cells and enzyme fuel cells. Today's more important fuel cells can be divided into several general categories, namely: (i) fuel cells utilizing compressed hydrogen (H2) as fuel; (ii) proton exchange membrane (PEM) fuel cells that use alcohols, e.g., methanol (CH3OH), metal hydrides, e.g., sodium borohydride (NaBH4), hydrocarbons, or other fuels reformed into hydrogen fuel; (iii) PEM fuel cells that can consume non-hydrogen fuel directly or direct oxidation fuel cells; and (iv) solid oxide fuel cells (SOFC) that directly convert hydrocarbon fuels to electricity at high temperature.
Compressed hydrogen is generally kept under high pressure and is therefore difficult to handle. Furthermore, large storage tanks are typically required and cannot be made sufficiently small for consumer electronic devices. Conventional reformat fuel cells require reformers and other vaporization and auxiliary systems to convert fuels to hydrogen to react with oxidant in the fuel cell. Recent advances make reformer or reformat fuel cells promising for consumer electronic devices. The most common direct oxidation fuel cells are direct methanol fuel cells or DMFC. Other direct oxidation fuel cells include direct ethanol fuel cells and direct tetramethyl orthocarbonate fuel cells. DMFC, in which methanol is reacted directly with oxidant in the fuel cell, has promising power application for consumer electronic devices. SOFC convert hydrocarbon fuels, such as butane, at high heat to produce electricity. SOFC requires relatively high temperature in the range of 1000° C. for the fuel cell reaction to occur.
The chemical reactions that produce electricity are different for each type of fuel cell. For DMFC, the chemical-electrical reaction at each electrode and the overall reaction for a direct methanol fuel cell are described as follows:
Half-reaction at the anode:CH3OH+H2O→CO2+6H++6e−
Half-reaction at the cathode:1.5O2+6H++6e−→3H2O
The overall fuel cell reaction:CH3OH+1.5O2→CO2+2H2O
Due to both the migration of the hydrogen ions (H+) through the PEM from the anode to the cathode and the inability of the free electrons (e−) to pass through the PEM, the electrons flow through an external circuit, thereby producing an electrical current. The external circuit may be used to power many useful consumer electronic devices, such as mobile or cell phones, calculators, personal digital assistants, laptop computers, and power tools, among others.
DMFC is discussed in U.S. Pat. Nos. 5,992,008 and 5,945,231, which are incorporated herein by reference in their entireties. Generally, the PEM is made from a polymer, such as Nafion® available from DuPont, which is a perfluorinated sulfonic acid polymer having a thickness in the range of about 0.05 mm to about 0.5 mm, or other suitable membranes. The anode is typically made from a Teflonized carbon paper support with a thin layer of catalyst, such as platinum-ruthenium, deposited thereon. The cathode is typically a gas diffusion electrode in which platinum particles are bonded to one side of the membrane.
In another direct oxidation fuel cell, borohydride fuel cell (DBFC) reacts as follows:
Half-reaction at the anode:BH4−+8OH−→BO2−+6H2O+8e−
Half-reaction at the cathode:2O2+4H2O+8e−→8OH−
In a chemical metal hydride fuel cell, generally aqueous sodium borohydride is reformed and reacts as follows:NaBH4+2H2O→(heat or catalyst)→4(H2)+(NaBO2)
Half-reaction at the anode:H2→2H++2e−
Half-reaction at the cathode:2(2H++2e−)+O2→2H2O
Suitable catalysts for this reaction include platinum and ruthenium, as well as other metals. The hydrogen fuel produced from reforming sodium borohydride is reacted in the fuel cell with an oxidant, such as O2, to create electricity (or a flow of electrons) and water byproduct. A sodium borate (NaBO2) byproduct is also produced by this process. A sodium borohydride fuel cell is discussed in U.S. Pat. No. 4,261,956, which is incorporated herein by reference. Therefore, the known chemical hydride reactions that use aqueous metal hydride have about 9 to 12 weight percentage storage expectancy, and the liquid and the catalyst used in the wet chemical reaction system need to be closely monitored. Additionally, it is difficult to maintain the stability of a metal hydride solution over a long period of time, because according to the formula t½−pH*log(0.034+kT), which provides the half life of the reaction, the reaction of hydrolysis always occurs very slowly. Furthermore, if the solution is stabilized, the reactivity is not complete.
In a hydride storage method, the reaction is as follows:Metal+H2→hydride+heat
However, storage expectancy of such a reaction is only about 5 weight percentage. Additionally, such reactions can be expensive and difficult to package.
Another known method to produce hydrogen is a dry hydride reaction. Dry reaction, generally, involves the following reaction:X(BH4)→H2, where X includes, but is not limited to, Na, Mg, Li, etc.
Again, dry reactions have several disadvantages, such as having a storage expectancy of only about 10 weight percentage, and the need to closely monitor the pressure.
An additional method to produce hydrogen gas is by a pressure storage method using the formula PV=nRT, wherein P is pressure, V is volume, n is a number of moles, R is the gas constant, and T is temperature. This method requires constant pressure monitoring.
One of the most important features for fuel cell application is fuel storage. Another important feature is regulating the transport of fuel out of the fuel cartridge to the fuel cell. To be commercially useful, fuel cells such as DMFC or PEM systems should have the capability of storing sufficient fuel to satisfy the consumers' normal usage. For example, for mobile or cell phones, for notebook computers, and for personal digital assistants (PDAs), fuel cells need to power these devices for at least as long as the current batteries and, preferably, much longer. Additionally, the fuel cells should have easily replaceable or refillable fuel tanks to minimize or obviate the need for lengthy recharges required by today's rechargeable batteries.
In the operation of a fuel cell, monitoring various system parameters in real time is highly desirable for a number of reasons. First, tracking the fuel usage history indicates the amount of fuel remaining in the fuel supply and provides the user with information regarding the remaining useful life of the fuel supply. The patent literature discloses a number of containers for consumable substances that include electronic memory components. U.S. patent application publication no. US 2002/0154815, which is incorporated herein in its entirety by reference, discloses a variety of containers that may include read-only memories, programmable read-only memories, electronically erasable programmable read-only memories, non-volatile random access memories, volatile random access memories or other types of electronic memory. These electronic memory devices may be used to retain coded recycle, refurbishing and/or refilling instructions for the containers, as well as a record of the use of the containers. The containers may comprise liquid ink or powdered toner for a printer. Alternatively, the containers or fuel supply may comprise a fuel cell or a fuel supply therefor.
Also, the transfer of the fuel from the fuel supply to the fuel cell may depend upon, inter alia, the viscosity of the fuel. For example, the viscosity of methanol, which is about 8.17×10−4 Pa-s at 1 atmosphere and 0° C., drops to about 4.5×10−4 Pa-s at 1 atmosphere and 40° C., representing about a 50% reduction. If the system is able to detect in real time the temperature and/or pressure of the fuel contained within the fuel supply, then the fuel cell can self-regulate how long a fuel pump should run in order to provide an appropriate amount of fuel. As fuel is supplied at the optimum rate, the efficiency of the system is increased. Also, monitoring the pressure of the fuel within the fuel supply can alert the user or the system of unacceptable high or unacceptable low pressure levels. Furthermore, the usable life of the fuel cell can be increased if exposure to fuel is limited to the amount of fuel necessary for operation. In other words, flooding the fuel cell with excess fuel may damage the fuel cell.
One option among others for a monitoring system is using a radio frequency identification (RFID) system. Systems using RFID technologies are well known, particularly for uses such as tracking inventory such as library or retail store inventory, automated payment systems such as passes for toll booths, and security systems such as smart keys for starting a car. Such systems may be large and active systems, utilizing battery-powered transceiver circuitry. Such systems may also be very small and passive, in which a transponder receives power from the base station or reader only when information is desired to be transmitted or exchanged.
A typical RFID system includes a reusable identifying device typically referred to as a tag, but sometimes designated as a “card,” “key,” or the like. The RFID system also requires a recognition or reader station that is prepared to recognize identifying devices of predetermined characteristics when such identifying device is brought within the proximity of the reader station. Typically, a reader station includes an antenna system that reads or interrogates the tags via a radio frequency (RF) link and a controller. The controller directs the interrogation of the tags and may provide memory for storing the data collected from the tags. Further, the controller may provide a user interface so that a user may externally monitor the data.
In operation, as a tag comes within sufficient proximity to an RFID reader station, the antenna emits RF signals towards the tag and the tag transmits responses to the antenna. The tags can be powered by an internal battery (an “active” tag) or by inductive coupling receiving induced power from the RF signals emitted from the antenna (a “passive” tag). Inductive coupling takes place between the two devices when they are proximate to one another; physical contact is unnecessary. Passive tags have zero maintenance and virtually unlimited life. The life span of an active tag is, however, limited by the lifetime of the battery, although some tags offer replaceable batteries.
Current monitoring systems with RFID tags have not been adapted for use with fuel cell systems, either in terms of the type of data desired to be monitored or in terms of the ability of the system to withstand the harsh environment due to contact with fuel cell fuels. It would, therefore, be desirable to provide an RFID monitoring system and other types of monitoring systems for use with a fuel cell system.